Oral dosage forms for the sustained release of drugs from water insoluble and slowly soluble matrices are well known in the prior art such as U.S. Pat. No. 4,389,393 and the release is known to occur by a diffusion process. As the diffusion path length increases with time, a linear plot of percent released versus t.sup.1/2 is obtained which is not ideal for maintenance of a plasma level which is intended to be more or less constant for an appropriate time.
Similarly, matrix systems usually exhibit an initial rapid release (but "burst" effect) of active ingredient which promotes increased plasma levels and may cause the adverse reactions which the dosage form was designed to minimize.
U.S. Pat. Nos. 4,505,890 and 4,610,870 describe controlled release formulations having a core containing gelling agent and a coat comprising either a film-forming agent and plasticizer or a hydrophilic polymer and a hydrophobic polymer. However in thos formulations, the core contains either from 8-14%, or from 5-15% of gelling agent.
With the present invention the "burst" effect has been eliminated by providing the matrix with a coating material that provides an additional barrier to diffusion of water into the matrix and drug solution out of the matrix to the external environment.